Inhibitory antibodies to factor VIII develop in approximately 30% of patients with severe and moderately severe hemophilia A in response to infusions of factor VIII. Inhibitor development is associated with a lower quality of life and an increased economic burden. Consequently, inhibitor development currently is considered the most significant complication of the management of in hemophilia A. Additionally, factor VIII inhibitors can occur in nonhemophiliacs, producing an autoimmune condition called acquired hemophilia A. Recent developments have made it possible to ask increasingly focused questions regarding the pathogenesis of factor VIII inhibitor development and to test novel diagnostic and therapeutic approaches to the problem. There are two Specific Aims in this project. In Aim 1, novel immunodominant epitopes recognized by anti-factor VIII antibodies produced in a murine hemophilia A immunogenicity model will be identified and characterized. This will be accomplished using several methods, including domain-specific antibody mapping, homolog and site-directed mutagenesis of factor VIII, and functional analysis of anti-factor VIII antibodies. Additionally, the pathogenicity of anti-factor VIII antibodies will be evaluated in hemophilia A mice. In Aim 2, murine antibodies of defined specificity produced in Aim 1 will be used to identify novel factor VIII antibody epitopes recognized by human hemophilia A patients.